During the lifetime of a patient, it may be necessary to perform a joint replacement procedure on the patient as a result of, for example, disease or trauma. The joint replacement procedure may involve the use of a prosthesis which is implanted into one of the patient's bones. In the case of a hip replacement procedure, a femoral prosthesis is implanted into the patient's thigh bone or femur. The femoral prosthesis is typically constructed as a one-piece structure having an upper portion which includes a spherically-shaped head which bears against the patient's pelvis or acetabulum, along with an elongated intramedullary stem which is utilized to secure the femoral component to the patient's femur. In order to secure the prosthesis to the patient's femur, the medullary canal of the patient's femur is first surgically prepared (e.g. reamed and/or broached) such that the intramedullary stem of the femoral prosthesis may be subsequently implanted therein. The femoral prosthesis may be press fit into the medullary canal or, in the alternative, bone cement may be utilized to secure the femoral prosthesis within the medullary canal.
During performance of a joint replacement procedure, it is generally necessary to provide the surgeon with a certain degree of flexibility in the selection of a prosthesis. In particular, the anatomy of the bone into which the prosthesis is to be implanted may vary somewhat from patient to patient. For example, in the case of a femoral prosthesis, the patient's femur may be relatively long or relatively short thereby requiring use of a femoral prosthesis which includes a stem that is relatively long or short, respectively. Moreover, in certain cases, such as when use of a relatively long stem length is required, the stem must also be bowed in order to conform to the anatomy of the patient's femur.
Such a need for prostheses of varying shapes and sizes this creates a number of problems in regard to use of a one-piece prosthesis. For example, a hospital or surgery center must maintain a relatively large inventory of prostheses in order to have the requisite mix of prostheses needed for certain situations such as trauma situations and revision surgery. Moreover, since the bow of the stem must conform to the bow of the intramedullary canal of the patient's femur, rotational positioning of the upper portion (i.e. proximal end) of the prosthesis is limited thereby rendering precise locating of the upper portion and hence the head of the prosthesis very difficult. In addition, since corresponding bones of the left and right side of a patient's anatomy (e.g. left and right femur) may bow in opposite directions, it is necessary to produce “left” and “right” variations of the prosthesis in order to provide anteversion of the bowed stem thereby further increasing the inventory of prostheses which must be maintained.
As a result of these and other drawbacks, a number of modular prostheses have been designed. As its name implies, a modular prosthesis is constructed in modular form so that the individual elements or features of the prosthesis can be selected to fit the needs of a given patient's anatomy. For example, modular prosthesis have been designed which include a proximal neck component which can be assembled to any one of numerous distal stem components in order to create an assembly which fits the needs of a given patient's anatomy. Such a design allows the distal stem component to be selected and thereafter implanted in the patient's bone in a position which conforms to the patient's anatomy while also allowing for a limited degree of independent positioning of the proximal neck component relative to the patient's pelvis.
One issue that arises as a result of use of a modular prosthesis is the locking of the components relative to one another. In particular, firm locking of the proximal neck component to the distal stem component is critical to prevent separation of the two components subsequent to implantation thereof into the patient. As such, a number of locking mechanisms have heretofore been designed to lock the components of a modular prosthesis to one another. For example, a number of modular prostheses have heretofore been designed to include a distal stem component which has an upwardly extending post which is received into a bore defined in the distal neck component. A relatively long fastener, such as a screw or bolt, is utilized to secure the post within the bore.
However, such a design has a number of drawbacks associated therewith. Firstly, functional loading during use of the prosthesis may not provide a positive lock and may actually tend to urge the upwardly extending post of the distal stem component out of the bore defined in the proximal neck component. In such a case, the fastener (e.g. the screw or bolt) alone must absorb such loads. This creates a number of problems since many of such functional loads tend to be axial in nature. In particular, by the nature of its design, axial loads exerted on a fastener such as a screw or bolt bear on the threads of the fastener thereby undesirably exerting a relatively large load to a relatively small surface area. Over time, such loads may degrade or even breach the mechanical integrity of the threads thereby potentially allowing the components to separate from one another.
Secondly, manufacture of such modular prosthesis is relatively difficult and, as a result, expensive. In particular, in order to utilize a long screw or bolt to secure the two components to one another, a relatively long bore must be drilled or otherwise machined through the entire length of the proximal neck component and at least a portion of the length of the distal stem component. Such drilling, often referred to as “gun drilling”, is relatively difficult to do since, amongst other things, it requires adherence to extremely strict tolerances thereby increasing costs associated with manufacture of the modular prosthesis.
What is needed therefore is a modular prosthesis which overcomes one or more of the above-mentioned drawbacks. What is particularly needed is a modular prosthesis which has enhanced locking characteristics relative to heretofore designed modular prostheses. What is further particularly needed is a modular prosthesis that is “self-locked” by the functional loads generated during use of the prosthesis.